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Sensors and detectors

Sensors and detectors. How to use sensor and detectors (in robotics ) RACE PROJECT VIGO (SPAIN) September 26-29, 2012. Definitions (1). Sensor: a device for sensing a physical variable of a physical system or an environment

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Sensors and detectors

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  1. Sensors and detectors Howtousesensor and detectors (in robotics) RACE PROJECT VIGO (SPAIN) September 26-29, 2012

  2. Definitions (1) • Sensor: a device for sensing a physical variable of a physical system or an environment • A sensor is in most cases associated with electronic circuits (converter) to generate a conditioned, normalized, amplified electric signal • A sensor plus a converter form a transducer, a device which transforms energy from one type to another (in this specific case electric energy) • Sometimes the sensor reacts by generating an electric signal itself, so it can be considered a transducer • Definitions of sensors and transducers do not always agree, so the word “sensor” alone might be enough to indicate these types of devices • Sensors are what the robotsneedtoknow the world … • fromhttp://www.robotshop.com/sensors.html

  3. Classifications 1 (physicalcharacteristic) • Primary (sensors/transducers) • Temperature • Light (photoelettric) • Strain gauge / Mechanicaltension • Magneticfield • Displacement (potenziometers) • … • Secondary (sensors/transducers) • Force, acceleration, pressure (based on strain gauge) • Displacement (based on photoelettric, magneticfield, capacitance, …) • Speed (based on displacement and time)

  4. Classifications 2 (output electricsignal) • Analog • The output isanelectricsignalwhichvariescontinuouslyaccordingto the variationsof the physicalvariablesbeeingmeasured • Digital • The output isanelectricpulsesignalwhich can assume onlytwovalues: logical 0 and 1. The frequency or the code associatedwith the pulsesequencecarries the information about the physicalvariablesbeeingmeasured

  5. Classifications 3 (energeticbehavior) • Active • Theyprovideanelectricsignalwhich can bedirectlyprocessedwithoutfurtherconsumptionofenergy: forexample the photovoltaiccells and termocouples • Passive • The requireanelectricalgenerator in ordertotransduce the physicalvariable in anelectricsignal: forexample the potentiometer

  6. Specifications (static and dynamicparameters) • Transfer function (transcaratteristic) • Monotone function • Linearity • Offset • Operationrange • Hysteresis • Sensitivity • Resolution • Repeatibility • Stability • Responsetime (timecostant e bandwidth) • Input and output impedance

  7. Specifications (parametri caratteristici statici e dinamici) Monotone function Linearity Sensitivity

  8. Sensors (in thispresentation) • Thermoresistance, thermistor, PN junction, integratedsensor (temperature) • Photoresistor, photodiode, phototransistor (light) • Tachometer (angolar speed) • Encoder (differenttipesfordisplacement and speed)

  9. Temperature - Thermoresistance (1) Metallicconductorswith a known “resistance vs temperature caracteristic curve”. The basicphysicalprincipleofthesedevicesisthat the electricconductivity (resistivity) decreases (increases) as the temperature increases. Thisholdstrueformaterialslikeplatinum, nickel, copper. The valueof T is in 0C If β and γ are smallcomparedto the valueofαthis relation can beconsideredlinear (forexampleplatinum) Ifnotitmightbenecessarytoperform a linearization (forexamplefor nickel and copperfortemperaturesabove 1000C) Thermoresistancehave a low sensitivity

  10. Temperature - Thermoresistance (2) Exampleof a temperature monitoring system from 00C to 3000C, output tensionbetween 0V and 10 V, based on PT100. VR V1

  11. Temperature - Thermistor NTC Unipolar semiconductor material The basicphysicalprincipleofthesedevicesisthat the electricconductivity (resistivity) of the pure semiconductor material increases (decreases) as the temperature increases. NTC (Negative Temperature Coefficient); T is in 0K Thermistors are very sensitive, but R isstrongly non linearwithrespectto T. Withhighlydopedsemicobductor material itispossibletoobtain PTC type (Positive Temperature Coefficient) thermistors

  12. Temperature - PN junction In a directbias PN junctionwithcostantcurrent the directdiodetensiondecreasesby 2,5 mVwith the increaseof 10C of the temperature. The exactvalueof the tensionfor a given temperature dependsupon the valueof the costantcurrentof the diode. A smallsignaljunctiondiodelike 1N914 or 1N4148 can beeasilyusedas a temperature sensor. Goodtimeresponse Calibrationisrequired

  13. Temperature - IC AD590 (1) Integratedsensor: Deviceswhichembed the sensor plus the circuitstonormalize, linearize, amplify the signal, in otherwords a “transducer”. The integrated T sensors are based on the lineardipendencebetween VD e T of the directbiascostantcurrentdiode (seeprevouspage). The junctionis the BE junctionof a BJT transistor AD590: high impedencecurrent generatori T is in 0K; K isμA/0K The generatedcurrentisdirectlyproportionalto the absolute T value It can belocated far from the measurementinstrument (itworkswithcurrent) and itisnotverysensitivtonoise It can show scale and offset errors The output currentsignalisconvertedtotensiomthrough a resistance plus a I/V converter (forexamplebased on OpAmp)

  14. Temperature - IC AD590 (2)

  15. Light sensor – Photoresistor (1) • Devices in which the information associatedto light isconverted in variationofresistance: resistivitydecreases (conductivityincreases) as the light increases • Madewith N typesemiconductore materiale (not a PN junction) • Thoughness, low priced, sensitivity • They can dissipatehighvaluesofpower (forexampletocontrolrelays) • Limitedbandwidth

  16. Light sensor – Photoresistor (2)

  17. Light sensor – Photodiode (1) • When a reversedbiased PN junctionisilluminated the total reverse currentisgivenby the sum of the typical revers current plus a componentproportionalto the luminousflux • Thesedevices are very fast and are highlyusedasdetectors in telecommunicationsystemsbased on fiberoptics

  18. Light sensor – Photodiode (2)

  19. Speed-Tachometergenerator (1) The tachometer generator (dynamo) is a small generator that produces an output voltage that is very accurately determined by its operating speed

  20. Tachometergenerator (2) • Simulation file

  21. Speed - Incremental encoder (1) Devicewhichmeasures the angolar displacementof a shaft in ordertogetinformationsabout the angularspeed (forexampleof a motor) Itismadeby a rotary disc and a Led/fototransistor system. On the circumferenceof the disc a set ofholeshasbeen set all at the samedistance on from the other. When the disc rotates the light beameitherisinterrupted (no hole and bjt in cut-off mode) or goesthroughfrom the led to the phototransistor (precenceof the hole and bjt in saturation mode). The phototransistorgenerates a trainpulse, onepulseforeachholecrossedby the light beam. From the numberofpulsesitispossibletodetermine the angulardisplacement and, in relation totime, the angularspeed With the incremental encoder itispossibletomeasure the speed, butitisnotpossibletodetermine the rotational direction

  22. Speed - Incremental encoder (2)

  23. Speed - Incremental encoder (2) INSERIRE LA FIGURA

  24. Speed - Incremental encoder (3) Two/threephasesincremental encoder todetermine the rotational direction

  25. Speed - Incremental encoder (4) Two/threephasesincremental encoder

  26. Speed – Absolute encoder (1) Usedtodetermine the shaftangular position. Eachcombinationofholesiscoded so toprovide the angular position of the disc. Normally the Gray code isused in ordertoprevent the transmissionoferrors WithsimplecombinatoryExorcircuititispossibletoconvertGray code in natualbinary code

  27. Motor controlwith encoder (1) Analogcontrol system

  28. Motor controlwith encoder (2) Digitalcontrol system

  29. Sensors(Society ofrobots) • Sensorsspecificforrobotics: Society ofrobots • http://www.societyofrobots.com/sensors.shtml

  30. Sensors in robotics & applications • Accelerometer • Color Sensors • DigitalCompass • Encoder (Slot, Rotary, Linear) • InfraredEmitter/Detector • Load and TorqueSensors • Mercury Tilt Switch • Photoresistor • Robot Computer Vision • SharpIRRangefinder • Sonar • Tactile Bumper Switch

  31. Sensors in robotics & applications • Accelerometer • Color Sensors • DigitalCompass • Encoder (Slot, Rotary, Linear) • InfraredEmitter/Detector • Load and TorqueSensors • Mercury Tilt Switch • Photoresistor • Robot Computer Vision • SharpIRRangefinder • Sonar • Tactile Bumper Switch

  32. Robot for the contest • Ideasfor the final meeting

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